When trying to access my Voicemail, I get the above message when using the default password 1234#. I also have no sounds when pressing the numbers on my phone while inputting the password (I do get button press sounds while dialing the voicemail number though. I have a DEFY XT I believe.
It will take a service ticket to get you back into your voicemail.
One thing you can try would be to disable WiFi before calling your VM. The Defy has a known issue in which digit tones are not always transmitted properly over WiFi and this could be the cause of your issue. If this doesn’t help there is also another known issue as @billg has pointed out and that one will require RW support to reset your pin.